1. Field of the Invention
The present invention relates to a semiconductor device and more particularly to improvement of a power semiconductor device formed using so-called wide-bandgap semiconductor such as silicon carbide (SiC), GaN, and diamond.
2. Description of the Background Art
A schematic block diagram in FIG. 10 conceptually shows an inverter used in motor control, which is disclosed in Japanese Patent Laying-Open No. 2006-073775 as an example of power module including power semiconductor devices according to a prior art.
As shown in FIG. 10, in a case where a semiconductor element (power element) for controlling a motor 31 or the like is included in an IC or a module, it is technically difficult to integrate in one chip or mount in the same package a control circuit 33 comprised of a low-voltage semiconductor circuit driven by a low-voltage power supply 32 of a control/logic system and power switching elements 35H, 35L operating with large current of high voltage HV supplied from a high-voltage power supply 34. Thus, there are few industrial ICs and modules used widely. In other words, power ICs and modules formed by the Si semiconductor technology at present are formed with quite complicated processes using an insulating technique for electrically isolating the low-voltage element and the high-voltage element from each other.
Particularly in a semiconductor device called Intelligent Power Module (IPM), a gate drive circuit 36 for high-side power switching element 35H of power switching elements (e.g., Insulated Gate Bipolar Transistors (IGBT) and Metal Oxide Semiconductor Field Effect Transistors (MOSFET)) in the inverter for controlling a motor 31 or the like should operate in a high-potential state floated from a ground potential, and a floating high-potential power supply 37 is then required.
The reason for this is that the potential at a connection portion between high-side power switching element 35H and low-side power switching element 35L connected to a load always varies depending on the states of these power switching elements and the gate potential for high-side power switching element 35H has to be supplied with respect to the varied potential in order to control the switching. Therefore, a level shift technique is required in which a signal on the basis of the ground potential sent from control circuit 33 is passed to gate drive circuit 36 in the high-potential floating state.
Typically employed as a level shift circuit for driving an Si power element in a conventional inverter is a scheme using a photocoupler as shown in FIG. 10. In this scheme, an LED (Light Emitting Diode) 39 emits light in response to a signal on the basis of the ground potential sent from control circuit 33, and a photodiode 38 is irradiated with the light. Then, the gate of high-side power switching element 35H is controlled depending on the signal of potential caused in photodiode 38 by the light irradiation. Use of a photocoupler in this manner allows a signal to be transmitted to gate drive circuit 36 in the high-potential floating state.
However, such a photocoupler is required for each high-side power switching element. In a three-phase output drive circuit in FIG. 10, for example, there are needed at least three photocouplers and three high-side gate drive power supplies. On the other hand, a gate drive circuit 40 for low-side power switching elements 35L is a low-voltage circuit and does not require such three independent floated power supplies as needed for the high-side. In other words, voltage from one low-side power supply 41 is supplied to three low-side power switching elements 35L by drive circuit 40 for inverter control.
As described above, the inverter as shown in FIG. 10 requires three power switching elements 35H, three gate drive circuits 36, three floating power supplies 37, and three level shift circuits (photodiodes 38, LEDs 39) on the high side and requires a certain capacity for mounting them, thereby disadvantageously increasing the size of the module containing them.
In view of the aforementioned problem in the power module shown in FIG. 10, Japanese Patent Laying-Open No. 2006-073775 proposes a method of improving heat resistance of a light-receiving element by forming both of a power switching element and the light-receiving element using wide-bandgap semiconductor, and also reducing a chip area by forming the power switching element and the light-receiving element on the same semiconductor chip.
However, in the method proposed in Japanese Patent Laying-Open No. 2006-073775, an LED for emitting light having a shorter wavelength than blue light has to be used to drive the light-receiving element, which causes increase in cost of the module.
More specifically, when a light-receiving element for producing a signal for driving a power semiconductor device is formed using a wide-bandgap semiconductor substrate, light having a wavelength shorter than the wavelength of blue light needs to be applied to excite the wide-bandgap semiconductor, as a signal light for exciting the light-receiving element. An LED that can emit light having such a short wavelength is generally fabricated using nitride semiconductor and is thus expensive, resulting in cost increase of the module.
Moreover, the wide-bandgap semiconductor element is liable to contain many defects, and it is difficult to control impurity diffusion for controlling electrical polarity. Therefore, it is also difficult to produce a high-performance light-receiving element with high sensitivity as well as good characteristics regarding such noise as dark current.